1. Field of the Invention
The invention relates to a high power continuous wave transverse gas dynamic laser system generally and to a plasma jet gas dynamic laser system specifically.
2. Description of the Prior Art
The optimum laser system is one which operates close to theoretical efficiency, generates a good quality laser beam (low divergence, single mode and stable position) and has practical operating conditions. An important factor in gas laser systems meant for industrial use is high overall power efficiency, which considers both the power for driving the laser and the power expended in pumping the gas through the system. Efficiency in a gas laser system relates to a high gain coefficient, an efficient exciting system, a moderate gas flow rate and a minimum of mirrors and windows. A high gain coefficient means that the volume of the active medium can be decreased, which in turn reduces wall deactivation losses and reduces the physical size of the system.
In a gas dynamic laser system the power output of the system is generally expressed as follows: laser power gained from excited flowing gas species equals power transmitted through mirrors plus mirror losses and diffraction losses. For a laser having a given active length, a gain medium of higher power density will have a higher value of power gained from the medium. Also, increasing the gain allows the mirror transmission to be increased. Specifically, power flux through a mirror is a product of the incident flux and the mirror transmission. Assuming a constant output flux, increasing the mirror transmission decreases the incident flux thereon and thereby minimizes the possibility of mirror damage and also minimizes mirror losses.
Mirrors impose an important design restraint on continuous wave carbon dioxide gas laser systems in that the maximum incident flux (I) they can accept without damage is approximately 5000 W/cm.sup.2.
The efficiency of a gas dynamic laser system utilizing an electrical discharge to excite the gain medium also depends on the efficiency of the electrical discharge in exciting the working gases of the system. The electrical discharge should be stable and operate at an electron temperature where the amount of energy transferred to the desired vibrational levels of the gaseous gain medium is close to the theoretical maximum. The discharge electrodes should be designed for minimal absorption of energy from the discharge due to vibrational wall deactivation.
The gaseous medium should gain only minimal translational energy from the discharge. Specifically, gaseous species having a high translational (kinetic) energy or temperature are an absorbing medium; whereas gaseous species having a high vibrational energy or temperature are an amplifying medium. Generally, the translational energy which a gaseous medium gains from an electrical discharge is directly proportional to the time the gaseous medium is in the discharge (residence time) and to the current density of the discharge.
The quality of an electrical discharge system for a gas laser can be described in terms of the specific power of the laser system. Specific power is defined as the maximum theoretical power obtainable from the laser system (P.sub.max) divided by the total mass flow rate of gases through the system (Q.sub.Tot). A specific value of translational energy increase is assumed. For example, if the electrical discharge creates gaseous species having a high kinetic or translational temperature, the actual specific power of the system decreases since such gas species comprise an absorbing medium in the laser system. Similarly, if the electrical discharge of the system creates gaseous species having a high vibrational temperature, the actual specific power of the system increases since such gaseous species are the gain medium.
An electrical discharge which produces excited gaseous species in which the translational (kinetic) temperature approaches that of the electronic or vibrational temperature is referred to as an "arc discharge," while an electrical discharge which produces excited gaseous species having a lower translational temperature than the electronic or vibrational temperatures is referred to as a "glow discharge." An "arc discharge" is undesirable in a gas laser system in that it produces an energy distribution unfavorable for lasing action in a gaseous gain medium. Furthermore, arc discharges are spatially inhomogeneous.
Another factor affecting the overall efficiency of a gas dynamic laser system is the flow rate of the gas through the system. Specifically, the power consumed by the pumps reduces overall efficiency of the laser system. Accordingly, it is desirable to have a minimum gas flow rate through the system in order to minimize pump power consumption.
Other desirable features of a gas dynamic flow laser system are as follows: homogeneous active medium, compact size, simple construction, reliability, stability, low operating voltage, high power density, metal construction and Gaussian gain distribution. Moreover, open circulation gas dynamic laser systems are preferred over closed circulation systems. Specifically, in an open system, the heated working gases are expelled into the atmosphere after the vibrational energy has been extracted, eg. a carbon dioxide - nitrogen laser system. Closed circulation systems are those in which it is either uneconomic or dangerous to expel the working gases into the atmosphere. Accordingly, in closed systems the working gases must be cooled, cleaned and then recirculated through the system, eg. carbon monoxide - nitrogen systems.
Transverse convective gas dynamic laser systems described in the prior art which utilize an electrical discharge to excite the gaseous gain medium can be divided into two basic categories: (1.) transverse electrical discharge systems and (2.) supersonic plasma jet electrical discharge systems.
Transverse electrical discharge systems basically comprise an electrical discharge maintained between two or more electrodes oriented variously with respect to gas flow. The lasing optical cavity is oriented transversely with respect to gas flow, either within the discharge region or immediately downstream from the discharge region. U.S. Pat. No. 3,721,915 issued to J. P. Reilly entitled "Electrically Excited Flowing Gas Laser and Method of Operation" describes a typical transverse discharge laser system.
The general disadvantages of transverse electrical discharge convective laser systems are as follows:
1. spatially uniform electrical discharges are difficult to maintain in a flowing gas environment; PA1 2. slight imperfections in the electrodes lead to arc formation and consequent kinetic heating of the gaseous medium; PA1 3. an extremely high voltage must be impressed across the electrodes to maintain the discharge; PA1 4. large flow rates of highly turbulent gas, with a consequent large pump power consumption, must be pumped through the discharge region to prevent arc formation.
Carbon dioxide - nitrogen convective laser systems have an additional disadvantage in that a large helium partial pressure in the working gas is required for discharge stability, temperature homogeneity and relaxation of the lower laser level, as well as other reasons. The helium component in the working gas induces pressure broadening of the gain coefficient, restricting it to low values. Further, the high cost of helium renders open cycle carbon dioxide - nitrogen convective gas laser systems impractical for most commercial applications.
Also, the power density of the gaseous gain medium in transverse electrical discharge systems generally is relatively low. Consequently, a relatively large number of mirror passes must be made across the optical cavity in order to extract significant power from the gain medium. Each mirror pass increases mirror losses, edge losses, and scattering losses.
Because of the foregoing disadvantages, transverse electrical discharge convective gas lasers have a relatively low overall efficiency and are not practical for most commercial applications.
Plasma jet electrical discharge laser systems described in the prior art utilize plasma jets which generate a gaseous plasma having a high kinetic or translational temperature as well as a high electronic or vibrational temperature. Specifically, the plasma jets described are arc plasma jets.
Typically, in the prior art plasma jet systems, the kinetic or translational temperature of the gaseous plasma is decreased by expanding it through a supersonic expansion nozzle and a lasing optical cavity is oriented transversely across the supersonic expansion nozzle to extract the vibrational energy from the expanding gaseous plasma. A typical supersonic plasma jet electrical discharge gas laser system is described in U.S. Pat. No. 3,811,095 issued to J. W. Rich et al. entitled "Electrical-Discharge Excited Gaseous Laser".
The disadvantages of supersonic plasma jet laser systems described in the prior art relate to the requirement for a supersonic expansion of the gaseous medium for reducing the kinetic or translational energy of the medium. Such supersonic expansion requires high gas flow rates with consequent increase in pump power consumption and decrease in overall efficiency. Furthermore, it is very difficult to design a supersonic expansion nozzle which would be structurally compatible with an optical lasing cavity oriented perpendicularly to such a nozzle. Specifically, such nozzles must be very carefully designed to prevent the formation of shock waves which would tend to refract the laser beam. Any apertures in the nozzle would tend to generate such shock waves.
Finally, it is very difficult to introduce and uniformly mix a lasing gaseous species into a gaseous plasma flowing at supersonic velocities. Consequently, in the prior art systems, the lasing gaseous species must be introduced and uniformly mixed into the exciting gaseous plasma before it reaches the supersonic expansion nozzle. Accordingly, the lasing gas species gain a substantial amount of translation or kinetic energy which again decreases the specific power of the system.
Because of the foregoing disadvantages, plasma jet convective gas laser systems tend to have low gain coefficients (ca. 1/m), low efficiencies (ca, 1%) and have not been deemed practical.